Method for wire electro-discharge machining a part

ABSTRACT

The present method for machining a part includes wire electro-discharge machining the part to create a recast layer, and then removing a zinc content of the recast layer without substantially altering the remainder of the initial composition make-up of the recast layer. The final composition make-up of the recast layer is substantially identical to the initial composition make-up except for the removed zinc content.

CROSS-REFERENCED TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/181,597 filed Jul. 29, 2008, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

The technical field relates generally to a method for wireelectro-discharge machining parts.

BACKGROUND OF THE ART

“Wire Electro-Discharge Machining” or “Wire Electric Discharge Machining(Wire-EDM) is a process that uses a wire electrode to machine metal.Wire-EDM may provide a highly accurate finished surface and a relativelyinexpensive process. However, Wire-EDM causes localized melting of thebase metal, which later solidifies into a recast layer. This recastlayer gives the Wire-EDM-ed material undesirable properties from afatigue life point of view. As such, when Wire-EDM has been used in theprior art, a secondary step involving the removal of the recast layer issometimes performed, which is time consuming and costly, and furtherusually results in dimensional discrepancies. Improvement is thereforedesired.

SUMMARY

There is accordingly provided, in accordance with one aspect, a methodfor machining a part, comprising: wire electro-discharge machining thepart using a wire electrode to create a recast layer having an initialcomposition make-up including a zinc content; and removing at least aportion of only the zinc content from the recast layer to produce afinal composition make-up of the recast layer, the final compositionmake-up being substantially identical to the initial composition make-upexcept for the removed zinc content.

There is also provided a method for machining a part, comprising: wireelectro-discharge machining the part using a wire electrode, the wireelectro-discharge machining creating a recast layer on a surface of themetal part, the recast layer having a zinc content; and selectivelyreducing the zinc content of at least a portion of the recast layerwhile leaving the recast layer otherwise substantially intact.

There is also provided a method for processing a wire electro-dischargemachined part, the part having a recast layer thereon produced by wireelectro-discharge machining of the part, the method comprising the stepof selectively reducing a zinc content in a recast layer formed by awire electro-discharge machining process, wherein a substantialremainder of the constituents of the recast layer are left on the metalpart after the performance of said step of reducing the zinc content.

There is also provided a method for producing a turbine disc of a gasturbine engine, the method comprising: removing material from theturbine disc by wire electro-discharge machining the turbine disc usinga wire electrode; and selectively reducing a zinc content of a recastlayer formed by the wire electro-discharge machining, while leaving therecast layer otherwise substantially intact.

There is further provided a turbine disc of a gas turbine enginecomprising: at least one blade root slot formed in the turbine disc andhaving a firtree profile created by wire electro-discharge machiningwith a wire electrode and having a treated recast layer, the treatedrecast layer having a lower zinc content than an original recast layerobtained following the wire electro-discharge machining, the treatedrecast layer remaining otherwise substantially intact and substantiallyidentical in make-up to the original recast layer composition.

Further details will be apparent from the detailed description andfigures included below.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures, in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic flow diagram of a method for machining a part;

FIG. 3 is a fragmentary cross-sectional view of a turbine disc having afirtree slot formed therein by Wire-EDM; and

FIG. 4 is a schematic partial cross-sectional view of a Wire-EDMproduced recast layer formed on the turbine disc of FIG. 3 .

DETAILED DESCRIPTION

Wire-EDM is a machining process in which material may be removed from apart (i.e. profiles may be formed in the part) by spark erosion causedby electric discharge between a wire electrode and the workpiece. Thewire electrode typically contains zinc, which is desired as an alloyingelement of the wire coating because it may provide higher cutting speedsand/or wire life. When Wire-EDM is used to cut through metal, alocalized melting of the base metal occurs, which later solidifies intoa so-called “recast layer”. The recast layer contains low melting-pointmetals which are transferred to the material of the cut metal part fromthe wire electrode used in the Wire-EDM process. However, this recastlayer may give certain undesirable properties to the Wire-EDM-edmaterial and, such as a reduced low cycle fatigue (LCF) life. Wire-EDMprocesses of the prior art, therefore, sought to remove this recastlayer completely from the machined part.

It has been observed by the present inventors, however, that only acertain constituent (i.e. element) of the recast layer, namely zinc, maycontribute to, or may itself be, the cause of one or more of theundesirable properties of the recast layer, such as fatigue lifereduction. Without intending to limit the scope of what is taughtherein, it is believed that zinc may act as an embrittling agent whendeposited on the surface of the part (i.e. in the recast layer) bydiffusing at high temperature in the high stress areas through grainboundaries. This may weaken the grain boundaries and initiateintergranular rupture from the surface. The synergetic effect of thedeposited zinc, the temperature, and the applied stress may therefore bethe features which induce low cycle fatigue (LCF) life reduction inparts such as Wire-EDMed gas turbine engine components, such as turbinediscs for example. Thus there may be stress, temperature, and zincconcentration thresholds required for the phenomena to initiate. Whenone of these factors is reduced, such as the stress or the zinc content,the material machined with Wire-EDM shows an extended LCF life.

Thus, removing zinc from the recast layer formed by the Wire-EDMprocess, either substantially entirely or partially, while leaving therecast layer otherwise substantially intact, may improve the propertiesof the Wire-EDMed part and increase its LCF life.

Therefore, after cutting and finishing by Wire-EDM as described infurther detail below, a step of “cleaning” (as defined herein) orprocessing the surfaces of the part is performed, and particularly therecast layer formed by the Wire-EDM, by using a process or means thatsubstantially eliminates or reduces to an acceptable level the amount ofdeposited zinc within the recast layer. Typically, the zinc amount isfound to be around 100 to 250 nm deep from the outer exposed surface ofthe part, which makes its elimination or reduction relatively easy andcan be carried out without otherwise removing the recast layer from thesurface of the part. In this description, it will be understood that,for brevity, the terms “removing”, “reducing” and variants thereof mayrefer interchangeably to mean either a partial removal or reduction ofzing content in the recast layer, or the substantially complete removal,reduction or elimination of zinc content from the recast layer.

In one embodiment, the zinc is removed from the recast layer of anexemplary alloy (such as Inconel™ or another Nickel supperalloy), usinga chemical process (i.e. a process which uses primarily chemical, andnot mechanical, means to remove zinc), such as by using a chemical bathas described below. However, other methods of removing or reducing thezinc content are also possible, such as by mechanically removing thezinc using a spindle deburring process, for example. In one example of asuch a chemical method used to reduce or substantially remove the zincfrom the recast layer and/or the rest of the part, the zinc is removedby immersing or plunging the Wire-EDM machined part, or at least aportion thereof including the recast layer, into a liquid solution thatspecifically targets the zinc such as at least reduce the zinc contentof the recast layer. Such a zinc-targeting solution may remove the zincfrom the recast layer by at least partly dissolving the zinc (whichtherefore remains in the solution) while leaving the other constituents(i.e. the composition) of the recast layer substantially intact. Inother words, the zinc is targeted and removed by the solution bydissolving or leaching the zinc out of the recast layer and into thesolution. Such zinc-targeting liquid solutions may include acidicsolutions, such as a nitric or phosphoric acid solution for example. Theterm “zinc-targeting” is used herein to mean any solution which may beused to remove (such as by dissolving, leaching or otherwise drawingout, for example) the zinc content from the recast layer. Acidic ornon-acidic solutions which dissolve zinc can be used. Any suitablesolution which reduces zinc content in the recast layer may be used.

In an embodiment, other constituent elements of the recast layer may beremoved with zinc, either simultaneously or successively, if desired.Thus, the zinc removal step may be selective (i.e. it removes only thezinc) as described above, or non-selective (i.e. removes zinc and otherconstituents which may be present). Preferably, of course, the solutionchosen to remove the zinc non-selectively does not remove and/ornegatively impact the material of the part itself.

Other suitable zinc removal processes may be used. For example, andwithout limiting to the teachings herein, since zinc typically onlyaffects a thickness of 100 to 250 nm from the outer surface of therecast layer (whereas the entire recast layer may be, for example, 5000nm (i.e. 0.005 mm) in thickness), a process that affects thiszinc-penetration depth of 100-250 nm may be used to remove the zinc,such as spindle deburring as mentioned above. Spindle deburring willonly remove the relatively softer material, such as zinc, from therecast layer. The spindle deburring process therefore modifies thesurface finish of the Wire-EDM surface, which includes at the same timethe reduction of the amount of zinc deposited therein.

Therefore, zinc may be targeted for selective removal from the recastlayer, or a given thickness (i.e. partial portion) of the recast layermay be removed (i.e. without removing the entire recast layer) in orderto ensure that the zinc within this removed partial portion of therecast layer, is also removed from the part and remaining recast layer.

As shown in FIG. 2 , a method for machining a part may include the stepsof: i) wire electro-discharge machining a metal (pure or alloyed) partwith a wire electrode, such as one which may contain zinc, the wireelectro-discharge machining creating a recast layer having a zinccontent; and ii) reducing the zinc content of the recast layer surfacewhile keeping the remainder of the recast substantially intact, whichmay include, in one example, keeping the composition make-up of therecast layer substantially identical except for the reduced zinccontent. A remaining portion of the recast layer may otherwise be lefton the surface of the part. The part being machined by Wire-EDM may be anickel-based superalloy, and the zinc from the recast layer formed bythe Wire-EDM process may be reduced or substantially eliminated bydipping the part, or at least a portion thereof including the recastlayer, into a solution that dissolves the zinc, such as a nitric orphosphoric acid solution for example. In one embodiment, the part isleft submersed in the zinc dissolving solution for a period of timeselected to correspond to a point at which substantially no further zincis removed from the recast layer into the solution.

Removing zinc from the recast layer allows one to avoid complete removalof the recast layer. This may improve the integrity of the part withouthaving to remove the entire recast layer.

Removing the zinc from the recast layer, without entirely removing therecast layer, may improve the fatigue life of the part which is machinedby a Wire-EDM process. As mentioned, zinc removal can be substantiallycomplete or partial. Removal can also be selective or non-selective,i.e. primarily only zinc may be removed or zinc may be removed withother constituents. The zinc content of the recast layer, following thezinc removal step, is lower than the zinc content of the original recastlayer, prior to the zinc removal step. The amount or percentage of zincremoved form the recast layer to achieve desired proprieties in aparticular application will depend on the pre-removal zinc content,parent material, stress and temperature applied to the part, partgeometry, etc., as will be appreciated by the skilled read in light ofthe present disclosure, and therefore there are processes parametersherein which may be tuned for use in each particular application. When achemical bath is used to remove zinc, a predetermined time of submersion(i.e. of the part in the nickel-dissolving solution bath) may thereforebe chosen based on the particular application, such that substantiallyno more zinc is removed from the recast layer beyond this period oftime. This time period may however be varied, depending on the amount ofzinc removal desired. For example, for lower expected temperature andstresses, less zinc may be removed to achieve the desired materialproperties of the part, and thus the submersion times may be shorter.

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, amultistage compressor 14 for pressurizing the air, a combustor 16 inwhich the compressed air is mixed with fuel and ignited for generatingan annular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases. Turbine section 18 includesat least one turbine disc 30, having a plurality of turbine bladesmounted thereto.

Referring to FIGS. 3 to 4 , an exemplary part machined by Wire-EDM is aturbine disc 30, in order to form the slots 40 having a “firtree”profile 45 in the turbine disc 30, the firtree profile slots receivingthe correspondingly shaped roots of the turbine blades therein. Sinceapplying Wire-EDM alone to make the firtree profiles or slots creates arecast layer at the surface of the workpiece which may decrease thefatigue life of the part, the making of the turbine discs according tothe teachings herein may be beneficial.

At least the firtree profiles 45, and/or the entire slots 40, may beformed in the turbine disc 30 using a Wire-EDM operation, which has theeffect of creating a recast layer 64 (as schematically seen in FIG. 4 ),which is typically 0.005 mm thick, and which may contain zinc that istransferred to the base material of the part being machined (i.e. thedisc 30) from the wire electrode used in the Wire-EDM process. If anyother low melting point materials (such as tin, lead, cadmium, mercury,silver, etc.) exist in the wire electrode used in the Wire-EDM process,these materials may also be transferred to the recast layer. Any ofthese low melting point materials, if they exist in the electrode andthus if they are transferred to the recast layer, may be enbrittlingagents for the material of the part being machined (such as nickel,aluminium and iron-base materials, etc.) by Wire-EDM. Thus, according tothe present method these low melting-point materials may also be removedfrom the recast layer, such as by immersing the part in a solution whichdissolves the targeted element or by other suitable processes.

Zinc may be removed from the recast layer 64, as described above, bysubmerging the section of the disc 30 including the recess 40 into afluid solution that dissolves zinc. Thus, the zinc content of the recastlayer 64 is at least reduced to a desired level, or substantiallyentirely removed, depending on an operational requirement of the part(e.g. such as desired LCF life) while leaving the remainder of therecast layer 64 substantially otherwise intact.

In one embodiment, the process of submerging the disc 30 (or other parthaving the recast layer) in the zinc-dissolving solution may include twoseparate submersing steps. The part is first submerged in a first tankhaving the zinc-dissolving solution therein, and the solution ismonitored and/or analyzed (i.e. controlled) to measure the amount ofzinc (i.e. the zinc level) in the solution, and therefore the amount ofzinc removed from the part. The zinc level in the solution is monitored,and the part is kept submerged in the zinc-removing solution until thezinc level rate of change drops below a selected threshold oralternately until the zinc-level rises to a selected maximum value. Thepart is then removed from the first tank, and subsequently submerged ina second tank, also containing a zinc-dissolving solution therein, whichmay be the same type of solution as in the first tank or a differentone. The zinc-dissolving solution in the second tank is similarlymonitored, such as to be able to determine the amount of zinc withinthis solution. This serves as a control and/or quality check method, inorder to determine whether all or most of the zinc has been removed fromthe recast layer. If the amount of zinc in the solution within thesecond tank does not significantly increase after the part is submergedtherein, then most or all of the zinc in the recast layer of the partmay have been removed.

In one example, a turbine disc made of the nickel superalloy IN100™ ismachined using a copper wire electrode coated with a copper-zinc alloyto form the firtree slots in the disc. The slots are then Wire-EDMfinished to a 32 Ra maximum roughness. The disc is then entirelysubmersed in a solution of 60% nitric acid for 15 minutes and thenremoved, and washed with water and air dried. To confirm that themaximum zinc removal has occurred, the disc is then submersed again(i.e. a second dipping step) in the same type of zinc-dissolving nitricacid solution, and the solution analyzed to ensure that no substantialremoval of zinc occurs in this second dipping step. This serves as anindication that the zinc has been substantially entirely removed fromthe recast layer.

It is of note that the method for removing zinc from the recast layer bysubmersing the turbine disc (or other part) in the zinc-dissolvingsolution may only remove the zinc content at is “free” within the recastlayer, and may not remove the zinc alloyed with other elements (such ascopper, for example), whether in the recast layer or the part itself.Without limiting the inventions claimed herein, it is believed that this“free” zinc may reduce the integrity (for example, the LCF life) of thepart if left in the recast layer.

The above-described process may be applied to any suitable partsmachined by Wire-EDM. Parts for a gas turbine engine which are LCF lifecritical, such as discs and shafts, may be particularly good candidatesfor this process, depending on the particular design considerations.Such parts which are W-EDMed using the above method may be made ofnickel superalloys, stainless steels or aluminum alloys.

It will be appreciated that when other materials, such as, for example,ME16™, Waspaloy™, and Inconel™ 718, are wire-EDM machined, several otherelements of the recast layer may be removed simultaneously whilereducing the zinc content.

It is understood that zinc-dissolving processes may not work as wellwhen used on parts made of low-alloy steels, as zinc-dissolvingsolutions may be harmful to this class of steels. However, the use ofmechanical or other means to reduce zinc, such as by mechanical removalof the zinc portion of the recast layer (e.g. by spindle deburring orother mechanical processes) may be used on such materials.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the inventions disclosed. For example,although turbine discs are described in the above example, it is to beunderstood that the present Wire-EDM method may be used to machine othergas turbine engine parts such as shafts, or non-gas turbine engine parts(i.e. parts used in other applications or industries), for example partswhich may have difficult geometry to machine and/or require very tighttolerances. Still other modifications which fall within the scope of thepresent invention will be apparent to those skilled in the art, in lightof a review of this disclosure, and such modifications are intended tofall within the appended claims.

The invention claimed is:
 1. A method for machining a part of a gasturbine engine, comprising: wire electro-discharge machining the partusing a wire electrode having a zinc component to create a recast layerhaving an initial composition make-up including a zinc content and atleast one other material content as a result of the wireelectro-discharge machining of the part; and removing at least a portionof only the zinc content from the recast layer without altering aremainder of the initial composition to produce a final compositionmake-up of the recast layer, the final composition make-up beingidentical to the initial composition make-up except for the removed zinccontent.
 2. The method as defined in claim 1, wherein removing the atleast portion of the zinc content from the recast layer comprisesremoving at least a portion of non-alloyed zinc content from the recastlayer.
 3. The method as defined in claim 1, wherein removing the atleast portion of the zinc content from the recast layer compriseseliminating the at least portion of the zinc content from the recastlayer.
 4. The method as defined in claim 1, wherein removing the atleast portion of the zinc content from the recast layer comprisesremoving the at least portion of the zinc content from an outer surfaceof the recast layer having a thickness of about 100 nm.
 5. The method asdefined in claim 1, wherein removing the at least portion of the zinccontent from the recast layer comprises removing the at least portion ofthe zinc content from an outer surface of the recast layer having athickness of about 250 nm.
 6. The method as defined in claim 1, whereinremoving at least a portion of the zinc content from the recast layerincludes submerging the part in a nitric acid solution.
 7. The method asdefined in claim 6, wherein submerging the part in the nitric acidsolution includes submerging the part in the nitric acid solution toremove the zinc content that is free within the recast layer, the nitricacid solution dissolving the zinc content that is free for removal froman outer surface and into the nitric acid solution.
 8. The method asdefined in claim 6, wherein submerging the part in the nitric acidsolution comprises submerging the part in the nitric acid solution for15 minutes.
 9. The method as defined in claim 6, further comprisingmonitoring a zinc level of the nitric acid solution, and keeping anouter surface of the recast layer submerged in the nitric acid solutionuntil a rate of change of the zinc level drops below a selectedthreshold.
 10. The method as defined in claim 6, further comprisingmonitoring a zinc level of the nitric acid solution, and keeping anouter surface of the recast layer submerged in the nitric acid solutionuntil the zinc level rises to a selected maximum value.
 11. The methodas defined in claim 1, wherein removing the at least portion of the zinccontent from the recast layer comprises mechanically removing a portionof an outer surface of the recast layer high in zinc content.
 12. Themethod as defined in claim 1, wherein the part is a turbine disc of agas turbine engine, and wherein wire electro-discharge machiningcomprises forming a firtree profiled slot within the disc.
 13. Themethod as defined in claim 1, wherein the part is composed of a nickelbased superalloy.
 14. The method as defined in claim 1, wherein the partis composed of a material selected from the group consisting of nickelbased superalloys, stainless steels and aluminum alloys.